I've been hiring caregivers for more than 30 years. Your mommy's sister is quite right; finding that special person to handle your AB needs can be a genuine struggle. I think the prospective mommy's expectations are the key consideration.
For example, I don't know how much she's accustomed to making per hour doing whatever sort of work she's been doing, but most ABs won't want to pay that much. I realize that's a ridiculous sentence, but it's the truth. The majority of ABs are ... well, parsimonious. Many don't make a lot of money and have little to spare, which is why they've never hired anyone to care for them.
For the healthcare professionals and nanny-types I've found, the going rate seems to be about $20-$30 per hour. Professional nannies with nursery furniture, equipment and supplies can charge several hundred dollars an hour, but getting the nursery together is not an investment for the faint of heart.
For the AB, caregiving is progressive rather than regressive ... once you have it, you want it again and again. This is a positive for your sister, if she can charge accordingly.
Assuming your mommy's sister doesn't intend to be a professional big babysitter, the other problem she will face is the selection of a venue. My current nanny bemoans the fact that there's no apartment or house nearby already set up as a big baby daycare. That means she has to come to me, or I have to go to her. Most of the time, she comes to me because I have a highchair and crib. I always meet any prospective caregiver at least twice in a public place before I ever give her my address, and I pay her for those meetings in addition to buying lunch, coffee, etc. If someone is going to provide intimate care for me, I want to make sure they're not a psychopath. The same is true for the sitter ... she'll want to carefully 'vet' prospective clients before she sees them.
I stay very busy and disruptions to my schedule are frequent. I pay my nanny for the time we would have spent together when I have to cancel. She wouldn't have thought of charging me when I call her two hours before a session and tell her my schedule has just been blown to hell; a good cancellation policy is something your mommy's sister will want to consider.
Advertising is crucial; many ABs are continually searching for a mommy. This requires more time and effort than money, but it's critical that the local AB community (such as it may be) knows she's available.
Finally, I'm not sure your mommy would be comfortable letting her sister 'practice' with you, but having some AB care experience before her sister starts charging for it will serve two purposes: It eliminates a learning curve and secondly, ensures that the prospective mommy is up for the job. I can't stress this point enough; nothing is more frustrating to both mommy and AB than for the mommy to have no clue what to do next. She'll need to have a mental 'care plan' and understand the roles she plays, and she'll need to be comfortable feeding and burping the AB (which often involves having the AB across her lap or over her shoulder), bathing an adult and changing diapers.
I'm not sure this is the sort of 'experience' you are looking for, but having had nearly 20 babysitters, I feel I'm something of an expert.